The present invention relates to a metering apparatus for introducing free-flowing, powdered substances in a controllable manner into spaces under pressure.
The said metering apparatus is intended to ensure troublefree addition of free-flowing, powdered substances, especially catalysts, into spaces under pressure, over long periods.
Catalyst metering by means of metering apparatuses used in practice can be divided into three process steps, ie. building up stocks of the catalyst, dividing the catalyst into portions and conveying the catalyst into reaction spaces under pressure. To avoid damage to the catalyst by catalyst poisons or agglomeration due to premature polymerization, the catalyst stock is blanketed with an inert gas. To regulate the amount of catalyst, an amount of catalyst defined by a geometrically specified volume is removed from the catalyst stock and then conveyed into the reaction space under pressure.
Apparatuses necessary for this purpose are known and are also described in the literature.
Four embodiments, described in four publications, are presented by way of example:
German Pat. No. 1,175,653 (I): Apparatus for batchwise metering of powdered substances, wherein metering into the space under pressure is effected by means of an oscillating, horizontally displaced pusher rod having a crosshole.
German Published Application DAS No. 2,062,513 (II): Apparatus for metering powders, wherein metering into the space under pressure is effected by means of an oscillating, vertically displaced plunger possessing a constriction.
German Laid-Open Application DOS No. 2,934,817 (III): Apparatus for introducing solids in a controlled manner into spaces under pressure, wherein metering into the space under pressure is effected by means of a rotating, horizontally arranged truncated cone whose conical surface contains depressions.
German Published Application DAS No. 2,264,412 (IV): This publication describes an apparatus by means of which the catalyst is released from a carrier chamber, as a dispersion or suspension, into the transporting gas, which then passes into the reaction space by a type of pneumatic transportation via a capillary tube.
However, the embodiments described have various disadvantages, which are described below.
In the embodiments according to publications I, II and III, reaction gas is used as a transport medium for the catalyst. Although the gas velocities in the particular transport lines are extremely high, premature polymerization in the transport lines results in these lines constantly becoming blocked. Because of these blockages, the metering apparatuses described frequently have only a short time-on-stream, with all the subsequent difficulties, such as shutting down the plant, cleaning the metering apparatuses, etc.
In the embodiments according to publication IV, the abovementioned difficulties are avoided by using an inert gas for transporting the catalyst. However, this transport gas must flow constantly through a narrow capillary tube at high velocity. A great disadvantage of this embodiment is that very small catalyst agglomerates or oversize particles cause blockages, which finally lead to the same difficulties as described above. Another disadvantage of this embodiment is that a large amount of inert gas is passed into the reaction space.